Re: A PVA for printing "gum" from Mike Ware

From: Katharine Thayer <kthayer@pacifier.com>
Subject: Re: A PVA for printing "gum" from Mike Ware
Date: Fri, 31 Mar 2006 07:41:49 -0800

> As to the latter, I'll quote the Bolte paper again (the
> introduction): "For a long time chromium (III) the final chromium
> species was thought to be responsible for the crosslinking process
> in dichromated systems. This assumption is quite logical with
> dichromated gelatin, chromium (III) being the only chromium species
> remaining after development. On the contrary, it is more surprising
> when dealing with dichromated PVA, a material in which chromium (V)
> has been proved to be stable..." I suppose it's statements like
> that which make me wonder if maybe the mechanism for gelatin might
> be different than that for PVA/gum.

I didn't read Bolte's 2005 paper, but in my understanding of her work
from a couple of years ago, she is recently interested in the behavior
of completely *dried,* dichromated *pure* PVA *film* with no
postexposure rinsing. If you read her papers carefully, you'll know
that moisture content of dichromated PVA or introduction of a few
percents of carboxyl groups (for example, by oxidation) can change the
results dramatically.

There is no good way to make a usable hologram from dichromated
gelatin without rinsing. Gelatin also contains significant fraction of
carboxyl groups. So in dichromated gelatin, Cr(V) is not found after
the exposure is turned off and the material is rinsed.

In the case of dichromated PVA *solution,* Cr(V) is rapidly dropped
after the exposure is turned off. This indicates that the presence of
moisture changes the reaction and Cr(V) becomes unstable; it's
converted to Cr(III). However, the measurement methods used by Bolte's
lab is not very efficient in detecting Cr(III) because it gives very
weak UV-vis spectroscopic signal at the amount sufficient to crosslink
the polymer, so absence of Cr(III) discussion/data in their paper does
not mean absence of Cr(III). Cr(III) can be measured more effectively
by low frequency dielectric loss, for example.

> [...] the fact that when I asked a question about gum crosslinking,
> Ryuji cited a paper about PVA crosslinking, suggests that he's
> probably in agreement with the former.

I think the reaction with gum is similar to not overly pure PVA (which
is what Bolte and others are using lately), or a blend of PVA and a
few percents of poly(acrylic acid).
Received on Fri Mar 31 10:51:49 2006